Background: HIV positivity and stigmatization among the mentally ill appears to be a complex subject of discuss among researchers. The prevalence of HIV among the mentally ill appears to be higher in some population and lower in others with varying patterns of stigmatization. Aim: This study determined the prevalence of HIV positivity and pattern of stigmatization among a population of mentally ill patients attending the University of Port Harcourt Teaching Hospital. Materials and Methods: This was a descriptive cross-sectional study conducted among attendees of Neuropsychiatric Outpatient Clinic of the University of Port Harcourt Teaching Hospital. A structured questionnaire covering socio-demographic characteristics and other factors were self-administered to 75 mentally ill patients attending the out-patient clinic following ethical clearance. Systematic random sampling technique was used to select the study participants. Descriptive statistics was used to analyze the data. Results: Majority of the respondents were male (81%), single (76.0%), and between the ages of 21 and 30 (44%). Majority (86.7%) had knowledge about HIV counseling and testing while those with no knowledge about HIV counseling and testing constituted 9.3%. Majority (76%) tested for HIV, while a minority (17.3%) did not. Within the last two years, majority (52%) had tested for HIV. Majority (25.3%) felt normal, 5.3% sad, while 1.3% felt very sad on learning about their HIV-positive status. HIV positive individuals, who have not been stigmatized because of their status constituted the majority (32%), while those that have been stigmatized constituted 4.0%. Conclusion: Majority of the mentally ill patients had knowledge about HIV counseling and testing, got tested, felt normal on learning about their HIV positive status, and were not stigmatized because of their status.